1. Field of the Invention
The present invention generally relates to a multiple subscriber communications system, and more particularly, to a spectral analysis of transmissions received from a communications medium.
2. Background Art
Communications networks often include a controller element that controls the allocation of resources on a network. For example, a Data Over Cable Based Communications System (DOCSIS), includes one or more headends, which control communications traffic originating from one or more subscribers (referred to herein as upstream traffic). System capacity allocated for upstream traffic is shared among multiple subscribers using capacity allocation schemes known as frequency division multiple access (FDMA) and time division multiple access (TDMA).
An FDMA system typically includes multiple frequency channels. Within an FDMA channel, upstream traffic transmission signals must conform to various requirements. Examples of these requirements include spectral mask limits, power limits, and spurious component limits. Frequency spectrum measurements are useful for determining whether such requirements are satisfied.
To optimally provide communications capacity for upstream traffic, it is desirable to obtain spectral information for each upstream transmission. Previous systems have employed swept spectrum analyzers to provide spectral information. Unfortunately, these analyzers have several disadvantages.
Two such disadvantages involve size and cost. Swept spectrum analyzers are typically bulky and expensive. Hence relatively few units can be provided in a receiver due to economical and size constraints.
Further disadvantages occur because a swept spectrum analyzer requires a time interval to “sweep” its analysis filter across a frequency band. Such time intervals may cause transient events to be missed that occur when the analysis filter is not tuned to the frequency where the event occurred. Examples of such transient events include impulse or burst noise.
Furthermore, since a swept spectrum analyzer sweeps its analysis filter across the band, it causes a linkage between the time and frequency domains. For this reason, it may be difficult to discriminate between the effects of transient time-domain and frequency-domain events.
In TDMA or SCDMA (Synchronous Code Division Multiple Access) systems, it is desirable to synchronize spectral analysis to particular transmissions that are scheduled into TDMA or SCDMA slots. Unfortunately, swept spectrum analyzers cannot be readily synchronized in this manner.
Accordingly, there is a need for techniques for obtaining spectral information for upstream transmissions that overcome the disadvantages and limitations described above.